Seems like everyone has one thing they can’t get enough of. For me, for about the first year, it was compasses. I resorted to buying some. The best offers I remember are during seasonal events.
The best free source for 3* unfarmable mats are titans. If you’re in C grade on 8* titans, you’re in Loot Tier VIII, which will give you 2 chances at ascension items. Moving up to Loot Tier IX will give you 3 ascension item drops, so increasing your odds by 50% of getting an unfarmable mat in titan loot
Increase loot tier by getting B grade or above, or by killing bigger titans.